Quality control of halftone screen printing

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Quality control of halftone silk printing although some factories have established standards and tolerances to guide production in specific production processes, the same standards do not exist for the silk printing industry. As a result, some silk printing houses tried to print halftone products without objective quality control. The result is that the accuracy is not enough to achieve the desired color. However, this is not the reason, because the silk printing process is controlled by a measurable range of changes, and its own color reproduction standards can be improved through measurement and use. In addition to helping you achieve accurate color on a constant basis, the establishment of quality control standards can also help manufacturers comprehensively improve the printing quality, improve the reproduction stability of the whole printing process and ensure the repeatability of the same or similar work. In the process of perfecting the standard, the most basic variables to be considered include the following: * printing point size and gradient range * ink layer thickness * optical density of printing color * point increase and point loss * ink overprint control the first two variables are mainly affected by the screen fabric used, and you will see that selecting the correct fiber diameter is as important as using the correct fabric and hole number. You can also learn how to calculate the tolerance related to these variables to ensure consistency with quality standards. The remaining variables are affected by other measurable features of screen making and printing. You will find out how to better evaluate these variables. Understand halftone point and tone range before evaluating point size and tone range, you need to understand the skills and screen parameters that affect them. The best place to start is with halftone images. Halftone is defined by the number of lines and the range of tones. The number of lines refers to the number of points per line inch or centimeter (line/inch, or line/centimeter). The higher the number of lines, the more points of each measurement unit, and the better the resolution of the image. The tone range is determined by the halftone dot size used to represent different levels of image density or ink range. For reproduction, the image is decomposed into yin and Yang dots of different sizes to represent brighter and darker areas. Each dot size represents the percentage of coverage from 0 to 100% (ratio of printed to non printed areas). For a specific halftone line number, the full frequency point size will produce the halftone tone tone range. This range includes highlights, intermediates, and darkens. For the% tone, use the positive image printing point to continuously increase, and the second tone continues to decrease from the% negative image point. (Figure 1) in silk printing, points less than 5% and more than 95% are usually discarded. Note that as the number of halftone lines increases, the dot size also increases (Table 1). This is an important basic principle, because points below a certain size will be lost, so it can not be reproduced in silk printing. As shown in Figure 2, the minimum highlight size that can be uniformly printed is limited by the hole fiber diameter. Because the dot ink cannot be guaranteed to fall on the open area of the hole, when the highlight is equal to or less than the fiber diameter, it cannot be printed. The printing ability of dark tone that hopes to bring help to the majority of users is also affected by the width of hole opening. When the darkened dot is smaller than the hole width, the template area containing the darkened dot will not adhere to the hole, and the dot will not be printed. (Fig. 3) the point size can be calculated at a specific tone value (f) based on the number of halftone lines. Simply use the following formula: 1 When halftone lines are given in centimeters, point size = ((1.1284 × Square root of F) ÷ number of lines/cm) × 1000 for example: calculate the size of a 48l/cm halftone point 5% --- point size = ((1.1284 × Square root of 5) ÷ 48) × 1000=52.6 micron 2 When halftone lines are given in inches, point size = ((1.1284 × Square root of F) ÷ number of lines/inch) × 2540 for example: calculate a 120l/in Halftone point of 5% point size - point size = ((1.1284 × Square root of 5) ÷ 120) × 2540=53.4 μ m hole fiber diameter relative to hole width also affects image printing performance. However, the fiber diameter listed in the technical data sheets of most silk manufacturers is a normal value, which represents the measured value before fiber weaving. In the process of weaving and perfecting, the circular cross-section of the fiber is deformed into a flat and oval shape, and the fiber diameter increases along the plane direction of the screen (Fig. 4). For the purposes of this article, I will call this wider fiber diameter the transverse fiber diameter. If the silk supplier provides relevant hole (MO) size data for a specific fabric, this information can be used to calculate the approximate transverse hole diameter using one of the following formulas: A. if the mesh is given in cm (mc/cm), the transverse fiber diameter = (10000 ÷ mc/cm) -mo B. if the mesh is given in inches (mc/in.), Transverse fiber diameter = (10000 × 2.54÷Mc/in.)- Mo for example, if you want to calculate the actual fiber diameter, 305 Wires/inch, a low draw rate wire with a normal diameter of 31 microns and a hole size (MO) of 48 microns, the formula will be expressed as follows: transverse fiber diameter = (10000 × 2.54 ÷ 305) -48=35.3 or 35 microns. The ratio of hole to transverse fiber diameter should be as high as possible. Screen fabrics with holes much larger than the transverse fiber diameter have less silk interference and easier ink circulation than those with smaller holes and thicker fibers. Therefore, they are more suitable for printing small dots. However, some work requirements may limit the fabric, where the hole width is less than or equal to the transverse fiber diameter. Regardless of the environment, the minimum highlight point of a given fabric can be calculated. 1. when the hole is larger than the transverse fiber diameter, the minimum point size = hole width + transverse fiber diameter 2 When the hole is equal to the fiber diameter, the minimum point size = (2 × Hole width) + transverse fiber diameter 3 When the hole is smaller than the fiber diameter, the minimum point size =2 × (hole width + transverse fiber diameter) in all cases, the printable darkening points must be equal to or greater than the hole width + fiber diameter. Because the smallest fiber diameter of today's leading silk fabrics is about 30 microns, the minimum printable highlights will be greater than or equal to 85 microns. Too small dot size will affect the printing quality and the consistency between printing and printing. Table 2 shows the minimum highlight and dim point sizes within a certain range of hole numbers and fiber diameters. Once the minimum size of highlights and darkening points supported by the screen fabric is determined, the maximum and minimum tone values that the main fabric will produce under a specific number of halftone lines can be calculated. The following formula can be used, where lc= number of halftone lines, mo= blank area, thd= transverse fiber diameter: 1 Minimum step value of printing highlight point = π × 100% × (printable point size × Lc)÷2)^2 2. Maximum step value of printing dark tone point =100- (π) × 100% × ((Mo+Thd) × LC) ÷ 2) ^ 2) for example, suppose you want to know the maximum and minimum tone values that can be reproduced by printing on a 305 silk/inch fabric with a half tone value of 85 threads/inch. Because in the previous example, the manufacturer provided a hole size of 48 microns and a normal fiber diameter of 31 microns. First, calculate the transverse fiber diameter (THD) as previously described, approximately equal to 35 microns. Next, determine the minimum point size. Since the hole is larger than the fiber diameter, the point size is equal to the sum of the hole and the transverse fiber diameter (35 μ m +48 μ m =83 μ m). Note that this value also represents the minimum dim point size. Finally, put these values into the minimum step value formula in sequence, and convert all values according to units (mm is used here): minimum step value = π × 100% × (0.083 × Under 3.346) ÷ 2) ^ 2=6.06% (≈ 6%), substitute the approximate value into the formula of maximum step adjustment value: maximum step adjustment value =100- (π) × 100% × ((0.083 × 3.35) ÷ 2) ^ 2) =93.9% (≈ 94%) for the printing resolution of the specified application, it mainly depends on the image size and observation distance. Table 3 lists the combination of ideal halftone line number and gradient range under different image sizes and viewing distances. With this table, you can select halftone and hole combinations suitable for printing. Table 3 image size and viewing distance affect the number of halftone lines and tone range suitable for a specific application. This table gives the ideal combination of image line number and gradient range for silk printing. Ink deposition thickness in addition to the print point size, the color of the printed image is also affected by the ink film thickness. Hole size, fiber diameter, template thickness, ink type and viscosity all affect the ink film thickness. We know that the wet thickness of ink deposition is equal to the theoretical color value of screen fabric, which is usually provided by the manufacturer. For example, a theoretical color value of 20 cm3/m2 will form a wet ink film thickness of approximately 20 microns. The ink film thickness can be calculated simply by dividing the color value by the hole area according to the following method: ((20 cm3 ÷ (100 cm × 100 cm)) × 10000 μ M/cm) =20 μ M. However, the template film thickness also affects the ink deposition, and the thickness must be increased according to the screen color value. Therefore, a wet ink film thickness of approximately 30 microns can be formed on a wire with a template thickness of 10 microns and a theoretical color value of 20 cubic centimeters/square meter. In order to print halftone better, the selection of screen fabric with low theoretical color value is as important as the use of thin film. Before establishing quality parameters, it is also necessary to measure template parameters, such as thickness and surface roughness. Once the acceptable values that are considered accurate, based on printing are determined, the screen making process should be set to ensure that the same template parameters can be obtained constantly. Optical density template control is particularly important when three primary colors are used. Because these inks are transparent to some extent (especially UV inks), the change of template can lead to the change of ink film thickness of printing ink, and then lead to the color deviation of printing color. This color deviation can be described by the light absorption of the ink film, which is called optical density. The densitometer can be used to measure the optical density of printing color. Densitometer is used to measure ink density in logarithmic form. Simply put, this value describes the ratio of the light absorbed by the "standard white" material to the light absorbed by the measuring material. "Normal inking" is a term used to describe the correct optical density for special ink/material combination printing. By recording the measured value of the densitometer of the printed matter, we can realize that the normal inking means the normal reproduction of color vision. In this way, when the same ink/material combination is used for printing, there is a software system of Jinan assay static load tensile testing machine that can be based on the multi line programming technology. Through this method, changes can be detected and the printed matter can be kept within an acceptable tolerance range. Point increase and loss point size change may be that the most common cable wire is not open circuit -- and it is difficult to be accurate -- resulting in inaccurate color. Any change in the point occurring before printing or during production will have a destructive impact on the color quality of the image. For this reason, monitor the point size of the whole production process and detect and correct any deviation found. During screen exposure, when a halftone point is transferred to the template, the point size will increase or decrease. The unexpected enlargement or reduction of the dot size between these films and the final print is often referred to as dot increase and dot loss, as shown in Figure 5. Note that the dot loss of highlights is usually called sharpening, while the dot loss of darkened dots is usually called paste. In most cases, enlargement and loss are usually caused by the following reasons: * due to incorrect selection of screen fabric (selection

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